The present invention relates to an electronic key telephone (EKT) system and, more particularly, to a control between a key service unit and key telephone or station sets included in an EKT system.
An EKT system allows a plurality of central office (CO) lines to be connected to a plurality of key station sets which belong to one subscriber. Each station set is capable of selecting any one of the CO lines or intercommunicating with another station set in the system. Such a system comprises, in addition to the station sets, a key service unit adapted to switch the lines, control the station sets, supply power and perform other various operations.
A station set offers various advanced services employing depression of buttons, flashing of lights and generation of an intermittent tone. For this purpose, the key service unit and each station set exchange with each other various data such as button data and lamp data in addition to speech communications. The key service unit and station set are, therefore, usually interconnected by two wires for communication exchange and two wires for data exchange, i.e. four wires in total. Power is supplied employing either one of the two sets of wires.
In the prior art EKT system, the key service unit scans a plurality of its associated station sets periodically at predetermined intervals so as to exchange data therewith. Each of the button on/off data and lamp on/off data has a string of bits which are allocated in a one-to-one relation to the buttons or the lamps, so that a bit position in the data designates a specific button or a specific lamp. Therefore, when the number of the buttons and/or that of the lamps is increased for more service features, the length of data exchanged between the key service unit and the key station sets will become longer and thereby susceptible to distortions due, for example, to the impedance in data paths. Moreover, an additional circuit will be required for compensating the data waveforms.
Meanwhile, the lamps at each station set is turned on and off directly controlled by the key service unit. Hence, they cannot flash at a frequency higher than the scanning frequency of the key service unit, that is, the scanning frequency cannot be determined regardless of the flashing frequency of the lamps.
A current trend in this art is, for advanced services, for the use of relatively simple character displays or the like in addition to the traditional lamps, loudspeakers and others available for display purpose on stations sets. The result would be a variety of modes of audible and visible display including different modes of lamp flashing, different frequencies and intervals of a tone and different characters indicated by the character display. Again, the prior art system which allocates display status data one bit to each display element is unsuitable for accommodating such various manners of display for the reason previously described.
In the prior art system, sources for generating a dial tone, a busy tone and the like are installed in the key service unit only, that is, the delivery of such tones to a station set is effected over a communication line. Therefore, when a call for a specific station set is received at one CO line while the specific set is communicating with another CO line, a dial tone representative of the receipt of the call cannot be sent to the specific station set.